Coelenterata is a term encompassing the animal phyla

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

(

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

animals, true jellies, sea anemones,

sea pen Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; 35 extant genera, and it is estimated that of 450 described species, around 200 are valid. Sea pens have a co ...

s, and their relatives) and

Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and ...

(comb jellies). The name comes , referring to the hollow

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...

common to these two phyla. They have very simple tissue organization, with only two layers of cells ( ectoderm and endoderm), and radial symmetry. Some examples are

s, which are typically colonial, and hydrae,

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrell ...

, and sea anemones, which are solitary. Coelenterata lack a specialized circulatory system relying instead on diffusion across the tissue layers.

Characteristics

All coelenterates are aquatic, mostly marine, animals. The body form is

radially symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

diploblastic Diploblasty is a condition of the blastula in which there are two primary germ layers: the ectoderm and endoderm. Diploblastic organisms are organisms which develop from such a blastula, and include cnidaria and ctenophora Ctenophora (; ...

and does not have a coelom. The body has a single opening, the , surrounded by sensory

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work ma ...

s equipped with either nematocysts or colloblasts to capture mostly

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

ic prey. These tentacles are surrounded by a spacious cavity called the

gastrovascular cavity The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Coelenterates or cnidarians (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into a ...

, or coelenteron. Digestion is both intracellular and extracellular. Respiration and excretion are accomplished by simple diffusion. A network of nerves is spread throughout the body. Many Cnidaria exhibit polymorphism, wherein different types of individuals are present in a colony for different functions. These individuals are called

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zoo ...

s. These animals generally reproduce asexually by budding, though sexual reproduction does occur in some groups.

History of classification

The scientific validity of the term ''coelenterate'' is currently rejected, as the Cnidaria and Ctenophora have less in common than previously assumed. Coelentera may only be monophyletic if both

Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', '' Hoilungia hongkongensis'', a ...

and Bilateria are included. In particular, the phylogenetic position of

is controversial; it was first considered a sub-group of coelenterata but Hyman regarded it as a separate phylum. Most researchers think that Coelenterata is not monophyletic, and therefore any group containing Cnidaria and Ctenophora but excluding other phyla would be paraphyletic. Previously, some genomic studies have found support for monophyletic coelenterates. Despite this uncertainty, the term ''coelenterate'' is still used in informal settings to refer to the Cnidaria and Ctenophora. Complicating the issue is the 1997 work of

Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...

(revising an earlier model by

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to discov ...

) that placed the Cnidaria and Ctenophora alone in the branch

Radiata Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of animals that do not form a monophyletic ...

within Eumetazoa. (The latter refers to all the animals except the sponges,

Trichoplax ''Trichoplax adhaerens'' is one of the three named species in the phylum Placozoa. The others are '' Hoilungia hongkongensis'' and '' Polyplacotoma mediterranea''. The Placozoa is a basal group of multicellular animals (metazoa). ''Trichoplax'' ...

, and the still poorly understood

Mesozoa The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. A recent study recovered Mesoz ...

). Neither grouping is accepted universally, however, both are commonly encountered in taxonomic literature.

References

{{Authority control Obsolete animal taxa Cnidarian taxonomy